What's recommended as the maximum stitch field size for designs on t-shirts across the chest?
It's not just the size of the design but how many stitches it has. A lot of stitches on a tee shirt may not look that good and can be uncomfortable to wear.
I only do LC designs on T's and even then I wouldn't do a stitch intensive one. If they need a full front then switch to a sweatshirt or have it printed.
I wouldn't try it. You would have to greatly decrease the number of stitches so you'll still have coverage problems. Additional backing on a T won't wear well.
Would use of a lower thread weight, like 30, and additional backing help at all?
To start with the weight of thread gets lighter as the number gets higher. 30 weight thread is thicker and heavier than 40 weight which is what most embroiderers use on a regular basis. 60 wt. thread is mostly used for small lettering and delicate designs.
If you used 60 wt. thread instead of 40 wt. you might wind up with gaps in the design. Smaller thread is going to give you smaller coverage.
A good digitizer can probably create an embroidery design that would work on a t-shirt and not weigh it down with too many stitches. There are lots of possibilities of making it work if have good artwork, good digitizing, proper backing, correct hooping, good thread and a little luck.
You still need to embroider the edges of the appliques so yes, the same restrictions still apply. You are limited by the hoop size on the machine unless you do them in multiple steps and the placements are independant.
max embroidery stitch field 
